This glass toggle is made from highly durable borosilicate glass. Borosilicate glass or 'boro' is created by adding boron to the traditional glassmaker's frit of silicate sand, soda and ground lime. The glass is commoly referred to as 'hard glass' because of it's durability and higher melting temperature. (Please note that sizing and coloring may vary from piece to piece as each item is hand made.)